On Thu, Oct 21, 2021 at 07:15:57PM +0200, Hans de Goede wrote:
On 10/20/21 23:14, Bjorn Helgaas wrote:
On Wed, Oct 20, 2021 at 12:23:26PM +0200, Hans de Goede wrote:
On 10/19/21 23:52, Bjorn Helgaas wrote:
On Thu, Oct 14, 2021 at 08:39:42PM +0200, Hans de Goede wrote:
Some BIOS-es contain a bug where they add addresses which map to system RAM in the PCI host bridge window returned by the ACPI _CRS method, see commit 4dc2287c1805 ("x86: avoid E820 regions when allocating address space").
To work around this bug Linux excludes E820 reserved addresses when allocating addresses from the PCI host bridge window since 2010. ...
I haven't seen anybody else eager to merge this, so I guess I'll stick my neck out here.
I applied this to my for-linus branch for v5.15.
Thank you, and sorry about the build-errors which the lkp kernel-test-robot found.
I've just send out a patch which fixes these build-errors (verified with both .config-s from the lkp reports). Feel free to squash this into the original patch (or keep them separate, whatever works for you).
Thanks, I squashed the fix in.
HOWEVER, I think it would be fairly risky to push this into v5.15. We would be relying on the assumption that current machines have all fixed the BIOS defect that 4dc2287c1805 addressed, and we have little evidence for that.
It is a 10 year old BIOS defect, so hopefully anything from 2018 or later will not have it.
We can hope. AFAIK, Windows allocates space top-down, while Linux allocates bottom-up, so I think it's quite possible these defects would never be discovered or fixed. In any event, I don't think we have much evidence either way.
I'm not sure there's significant benefit to having this in v5.15. Yes, the mainline v5.15 kernel would work on the affected machines, but I suspect most people with those machines are running distro kernels, not mainline kernels.
Fedora and Arch do follow mainline pretty closely and a lot of users are affected by this (see the large number of BugLinks in the commit).
I completely understand why you are reluctant to push this out, but your argument about most distros not running mainline kernels also applies to chances of people where this may cause a regression running mainline kernels also being quite small.
True.
This issue has been around a long time, so it's not like a regression that we just introduced. If we fixed these machines and regressed *other* machines, we'd be worse off than we are now.
If we break one machine model and fix a whole bunch of other machines then in my book that is a win. Ideally we would not break anything, but we can only find out if we actually break anything if we ship the change.
I'm definitely not going to try the "fix many, break one" argument on Linus. Of course we want to fix systems, but IMO it's far better to leave a system broken than it is to break one that used to work.
In the meantime, here's another possibility for working around this. What if we discarded remove_e820_regions() completely, but aligned the problem _CRS windows a little more? The 4dc2287c1805 case was this:
BIOS-e820: 00000000bfe4dc00 - 00000000c0000000 (reserved) pci_root PNP0A03:00: host bridge window [mem 0xbff00000-0xdfffffff]
where the _CRS window was of size 0x20100000, i.e., 512M + 1M. At least in this particular case, we could avoid the problem by throwing away that first 1M and aligning the window to a nice 3G boundary. Maybe it would be worth giving up a small fraction (less than 0.2% in this case) of questionable windows like this?
The PCI BAR allocation code tries to fall back to the BIOS assigned resource if the allocation fails. That BIOS assigned resource might fall outside of the host bridge window after we round the address.
My initial gut instinct here is that this has a bigger chance of breaking things then my change.
In the beginning of the thread you said that ideally we would completely stop using the E820 reservations for PCI host bridge windows. Because in hindsight messing with the windows on all machines just to work around a clear BIOS bug in some was not a good idea.
This address-rounding/-aligning you now suggest, is again messing with the windows on all machines just to work around a clear BIOS bug in some. At least that is how I see this.
That's true. I assume Red Hat has a bunch of machines and hopefully an archive of dmesg logs from them. Those logs should contain good E820 and _CRS information, so with a little scripting, maybe we could get some idea of what's out there.
Bjorn